The epidemic growth of severe obesity continues unabated, and 36 million U.S. adults (14.5%) currently have a BMI =35 kg/m2. In the last 20 years, bariatric procedures have emerged as the most effective interventions to induce weight loss and improvements in comorbidities. However, evidence about the long-term outcomes of bariatric surgery remains a critical gap. The proposed project will be the largest study to date to investigate the long-term relationships between bariatric surgery and durable improvements in body weight, hypertension, and renal disease, as well as its association with major operative complications and long-term survival. We will also compare these outcomes to a population-based cohort of severely obese patients who have not had weight loss surgery and investigate demographic and clinical characteristics (e.g., age, gender, race/ethnicity, bariatric procedure type, T2DM severity, and other comorbid medical and mental health conditions and their treatments) that may be associated with beneficial and adverse outcomes of surgery. Building upon methods that have been successfully applied in our prior studies, we will conduct a retrospective cohort study of more than 45,000 severely obese patients who underwent bariatric surgery in three large integrated health care systems over a 17-year period (1997-2013). This is the largest and most diverse bariatric population in the world with 41% racial/ethnic minority patients. Bariatric procedures and long-term outcomes will be identified from existing electronic medical record and registry databases using approaches that have been previously published or validated by members of our team. We will use advanced propensity- based statistical approaches to carefully identify a non-surgical comparison group with severe obesity. And we will investigate the potential for biases in our databases using a combination of primary data collection and chart review. These novel data will allow us to establish the validity of our outcome measures and establish whether subjects who have missing data in our databases have different long-term outcomes than those with complete data. The information gained from this study will enable physicians and patients to more accurately assess the long- term risks and benefits of bariatric surgical intervention and will help guide improvements in pre- and post- operative care pathways for bariatric patients. Our findings will also help improve the return on investment that healthcare systems are making in weight loss surgery.